Missiles, aircraft, space systems and other vehicles commonly require multiple pyrotechnic devices, including destruct units, initiators, severance systems, actuators, etc. Due to the volatile nature of such pyrotechnic devices, most require safety features to prevent their inadvertent and hazardous initiation during maintenance of the vehicle prior to a mission. Such safety features are commonly known as safe and arm units, two of which are shown in U.S. Pat. No. 3,728,936 to Norris and U.S. Pat. No. 4,202,271 to Day.
Many pyrotechnic systems rely on what are generally known as linear explosive products or detonation transfer lines to communicate a signal from an explosion initiator to an output device on the vehicle. Such detonation transfer lines are used in place of electrically conductive wires, or the like, to ensure detonation of the pyrotechnic device in environments where large or fluctuating electromagnetic fields may render reliability of conductors suspect. The detonation transfer lines are typically thin-walled metal tubes with an optional outer braid, the tubes being packed with explosive material. These lines act as fuses in a sense, except they do not "burn" but rather detonate and propagate the percussive blast at a velocity of up to 7,000 meters per second. Two conventional detonation transfer lines are SMDC (Shielded Mild Detonating Cord) and FCDC (Flexible Confined Detonating Cord). These types of detonation transfer lines have a relatively mild concentration of explosive, such as RDX, along their length to prevent rupture of the enclosing tube.
It is vital to prevent the output device from being inadvertently initiated while ground personnel are working on the vehicle. In this respect, the safe and arm device prevents the initial explosion from propagating down the detonation transfer line from the initiator to the output device. Commonly, the safe and arm device utilizes a solenoid-driven barrier to physically block the explosive output from the initiator. In the armed mode, an air gap is typically formed by aligning a barrier aperture between the initiator output and the ordnance transfer line. With the barrier aperture aligned in this manner, the percussive force generated by the initiator propagates across the air gap to continue along the transfer line to the output device.
Due to the mild concentration of explosive material, the air gap must be quite small. Prior testing by others has indicated that detonation transfer lines can propagate across an open air gap of up to 5 inches, yet only a maximum of 0.25 inches reliably across confined gaps. This short propagation distance in confined air gaps has proved to be a detriment in the design process. While linear-actuated barriers and disc-like rotary barriers may be quite thin, and thus allow for very small air gaps, they require a substantial amount of space to operate, which is a drawback in compact vehicles such as missiles.
There are several drawbacks to conventional safe and arm devices. Primarily, it is less than desirable to manufacture, ship and install a safe and arm device containing an initiator or an internal explosive, due to the chance of inadvertent detonation. Additionally, there is typically one output device per initiator and associated safe and arm device. Combining two such devices into one is complex and can increase the cost substantially, especially in vehicles with multiple pyrotechnic systems. It is apparent there are drawbacks with present barrier-style safe and arm devices containing initiators.
A further feature desired by most customers for safe and arm security devices within pyrotechnic systems is a safety key which can lock the device in a safe position prior to the intended mission to ensure that output devices are not initiated. In the event an arm signal is sent while the particular device is in the locked safe mode, the safety key cannot then be withdrawn before an arm signal is removed.
In barrier-style safe and arm devices, the safety key serves to physically lock the barrier in a position closing the air gap leading to the transfer line. Prior mechanisms for preventing removal of the safety key in the event of an erroneous arm signal have made use of a separate solenoid-driven locking element or a relatively complex bolt or other locking arrangement. These mechanisms ultimately add weight and cost and reduce the reliability of the overall system.
The extreme shock, acceleration or vibratory motions imposed on vehicles in flight can inadvertently actuate mechanical devices such as barrier-type safe and arm devices. Just prior to a mission, the safety key is removed and the device then is capable of being armed, depending on remote signals from a control unit or operator. However, the internal mechanism for translating or rotating the barrier is typically biased in one position or the other without rigid physical impediments to motion, thus allowing the possibility of an unwanted position change due to external forces. Such an unwanted change from an arm to safe position, for example, is highly undesirable.
In short, there exists a need for a compact safe and arm device utilizing a safety key which overcomes the drawbacks of the prior art.